Both low density polyethylenes (LDPEs) made using a conventional high-pressure process and linear low density polyethylenes (LLDPEs) produced using a Ziegler-Natta catalyst in a gas phase process, and blends and articles made therefrom, are generally known in the art. While such polyethylenes are sometimes preferred because they provide relatively low-cost solutions to a number of needs, their properties render them less desirable than other polyethylenes for a number of applications. For example, LLDPE and LDPE films cannot be produced in high-stalk bubble blown film lines due to a lack of melt strength, and therefore cannot be produced with balanced machine direction (MD)—transverse direction (TD) shrink properties. Also, gauge films cannot be produced with LLDPE in a low-stalk process with a reasonable line speed due to a lack of melt strength.
Certain metallocene-catalyzed polyethylenes are able to be used in a high-stalk process and in producing thick gauge films in a low-stalk process because of the melt strength they exhibit, and are also able to contribute desired toughness and optical properties to the films that are produced therewith. These metallocene-catalyzed polyethylenes are able to produce film with high TD shrinkability even at thin gauge, good toughness (particularly as measured by, e.g., MD tear, dart and puncture resistance) and good optical properties (e.g., low haze and high gloss).
However, metallocene-catalyzed polyethylene sometimes require a higher melt strength, and attempts have been made to blend it with polypropylene with that goal in mind. As a result, it has been observed that while polypropylene blended into metallocene-catalyzed polyethylene may provide additional melt strength, there is often poor miscibility between the two resins which may cause undesirable mechanical and optical properties. It has also been observed that there is an additional need for polyethylene-polypropylene blends that exhibit increased elasticity and processability.
Background references include WO 2007/003523 and EP 0 757 069 A.